Particulate removal system

ABSTRACT

A particulate removal apparatus and method are used to capture and remove particulates from nuclear reactor core coolant during normal operation. Bottom nozzle, particulate removal apparatus and top nozzle structures form an assembly sized to be installed in place of a nuclear fuel assembly. The particulate removal achieved reduces the inventory of corrosion product deposits, foreign objects and other particulates in the reactor coolant system. This in turn reduces activation or deposition of particulates on fuel cladding, with a corresponding improvement in fuel reliability and reduction in ex-core radiation fields.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority and benefit under 35 U.S.C. §119(e) toU.S. Provisional Patent Application No. 61/569,631, entitled “In SituCore Filter Module,” filed on Dec. 12, 2011. The content of thatapplication is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

This invention relates to improving the safety, reliability andperformance of nuclear reactors. More specifically, the inventionrelates to a method and apparatus for improving conditions in theprimary system of light water reactors by removing impurities that mayotherwise become activated in the reactor core and deposit on surfaceswithin the primary circuit, leading to component degradation andincreased dose rates. The invention is applicable to pressurized waterreactors (PWRs) and boiling water reactors (BWRs).

PWR nuclear power reactors use recirculating subcooled water in thereactor coolant system (RCS) to remove energy produced by fission in thecore. The recirculating water flows upward under pressure through thecore and then to the “primary side” of one or more steam generators,wherein the energy is transferred through the steam generator tubes tothe secondary side of the steam generators where water is boiled toproduce saturated or superheated steam. Most of the steam produced inthe steam generator is directed to a turbine generator to produceelectricity. Some steam is used to reheat steam in the secondary cycle,drive steam turbine driven pumps, or preheat feedwater that is fed tothe steam generators. In some plants, steam may be used for otherpurpose such as seawater desalinization.

BWR nuclear power reactors use recirculating water to remove energyproduced by fission in the core but unlike in PWRs, pool boiling occurswithin the core of the reactor. To maintain favorable heat transfer andcontrol of the fission process in BWRs, one or more recirculation loopsare used to forcibly circulate liquid water upward through the core.Favorable conditions include: (1) enhanced convective and boiling heatflux at the higher velocities produced by recirculation, and (2) ahigher liquid water fraction which increases moderation of neutrons.Steam generated in the core is separated from the recirculatingsteam-water mixture produced in the core and directed to a turbinegenerator to produce electricity, secondary cycle steam reheaters, steamturbine driven pumps, or to feedwater heaters to pre-heat recycledfeedwater. The liquid phase water that exits the core with the steam isseparated from the steam and is pumped back into the lower end of thecore with the recirculation pumps. The recirculation pumps may be motordriven centrifugal pumps or a combination of jet pumps and centrifugalpumps.

Nuclear power reactors generate heat in their cores by fission offissile materials such as U-235 or Pu-239. The cores may also containfertile materials such as Th-233, which can be converted to fissionablespecies by irradiation in the core. The concentration of fissilematerial in PWR or BWR fuel is typically enriched over what is found innature. The enrichment is typically to 2 to 20%, but it may be muchhigher. The balance of the fuel is typically naturally occurringnon-fissile material (e.g., U-238). In light water reactors, theneutrons produced by the fission process are “moderated” by the water.Moderation lowers the energy of the neutrons and renders them morelikely to promote desirable fission chain reactions with the fuel.

The chemical form of the fuel in most reactors is solid uranium oxide ora mixture of uranium oxide and plutonium oxide, but other forms that maybe used include uranium or plutonium solid metal alloys. In general, theoxide fuel is formed into cylindrical pellets, which are stacked in fuelrods within “cladding” and further grouped in fuel rod assemblies. Mostcladding is fabricated from zirconium alloys owing to the transparencyof zirconium to neutrons, known as low neutron cross section, as well asthe good corrosion resistance of zirconium alloys. A typical PWR maycontain about 200 fuel assembles, each of which contains about 250 fuelrods (or pins) that are 3 to 5 meters in length. A typical BWR maycontain 600 to 800 fuel assemblies, each of which typically contains 60to 100 fuel rods (or pins) that are 3 to 4 meters in length. BWR fuel isalso typically housed in removable “channels” which are elongated squaretubes. The main purpose of the channel is to prevent cross flow of waterand steam from assembly to assembly which further serves to ensurefavorable thermal hydraulics, heat transfer and control of the fissionprocess. In PWRs, cross flow of water from assembly to assembly is notavoided; therefore, fuel rods are not channeled but distributed in anopen square or triangular pitch array.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of a PWR primarycoolant loop;

FIG. 2 is a plan view of an embodiment of a reactor vessel core,including fuel assemblies within a circular envelope

FIG. 3 is a plan view schematic of an embodiment of a particulateremoval apparatus;

FIG. 4 is an elevation view of an embodiment of a particulate removalapparatus with a single zone of filtration;

FIG. 5 is an elevation view of an embodiment of a particulate removalapparatus with multiple zones of separation within filtering region

FIG. 6 is an elevation view of an embodiment of a particulate removalapparatus with multiple zones of separation within the filtering region;

FIG. 7 is an elevation view of an embodiment of a particulate removalapparatus illustrating the bypass flow path through the apparatus;

FIG. 8 is a plan view schematic of an embodiment of a particulateremoval which includes a number of fuel rods in addition to thefiltration region;

FIG. 9 is an elevation view of an embodiment of a particulate removalapparatus in which filtration elements comprise only a portion of theoverall length of the apparatus; and

FIG. 10 is a graph illustrating theoretical accumulation of particulatesin the core (crud) over a typical operating cycle of 12 to 18 months vs.efficiency of particulate removal in accordance with an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

In both PWR and BWR cores, about one-third of the fuel assemblies arereplaced each refueling outage after a 12 to 24 month operating cycle.New fuel loaded into electric power generating PWR and BWR reactors maycontain enrichment in fissile species up to 5% or so. The fuel that hasbeen in the core for one cycle contains less fissile material, on theorder of about one-half to one-third of the original fissile material.The fuel that has been used twice but is re-loaded in the core for athird cycle may contain even less fissile material, as little as 0.1 to1% fissile material after two cycles of use. In fact, the initialfissile material in some of the oldest fuel may contribute less than0.02% to the output of the plant (per assembly) as compared to anaverage of 0.5% (per assembly) for a core with 200 assemblies. In otherwords, these highly depleted assemblies may contribute as little as1/25th of energy of a new fuel assembly based on the initial fissilematerial enrichment. This low contribution is offset by the fission offissile materials produced in the fuel during prior cycles of operation,such as Pu-239 and Pu-241 that can form from U-238 in PWR fuel.

Some older fuel, or so called “barrier assemblies,” are placed at theperiphery of the reactor core primarily as a neutron shield for thereactor vessel and internals to mitigate irradiation induced stresscorrosion cracking of internals or embrittlement of the reactor vesseldue to high neutron fluence (see for example NUREG 1.99 Revision 2). Thetwice burned fuel is often used as a shield owing to the significantmass of high density fuel materials, and hence a high affinity forabsorbing radiation and neutrons. In some PWR core design strategies, onthe order of 4 to 8 assemblies may be used as barriers at the peripheryof the core. The reactor coolant flow through each of the fuelassemblies in a typical PWR core, including the barrier assemblies, isabout 0.5% of the total flow. Four barrier assemblies in a PWR wouldreceive in total about 2% of the RCS flow.

The weight of a typical PWR fuel assembly is about 1200 pounds (0.75metric tonnes). The weight of a typical BWR assembly is about 550 pounds(0.5 metric tomes). During normal operation, the drag forces on fuel dueto fluid flow are comparable to the weight of the fuel so there are nounacceptable upward drag loads on the upper core supports.

In a PWR, the RCS operates at about 2000 to 3000 psi (140 to 200 Bar)and 550 to 625° F. (285 to 330° C.) (subcooled conditions). In a typicalBWR, the RCS operates at 1000 to 1100 psi (68 to 75 bar) and 550° F.(28.5° C.) (saturated conditions). The pressure drop through the core ina PWR is typically 25 to 75 psi (1.7 to 5 bar), which combined withother pressure losses in the reactor vessel, steam generators and RCSpiping is overcome with reactor coolant pumps. The average pressure dropacross the core of a BWR is on the order of 25 psi (1.70 Bar). In bothPWR and BWR fuel designs, a series of open lattice support “grids” arealso used to support the fuel rods that make up the fuel bundle,maintain separation between the rods and suppress vibration due to flowof the water or steam-water mixture along or across the rods. Thesegrids may be fabricated from zirconium alloys or other metals. At thetop and bottom of the fuel assemblies, upper and lower “nozzles” or tieplates structurally support the fuel, and engage with the lower andupper core support plates in the reactor vessel. The upper and lowernozzles are typically fabricated from stainless steel. The upper andlower core support plates in the reactor which support and engage thefuel are part of the overall reactor vessel internals arrangement.

PWR and BWR fuel assemblies may also contain other features includingbut not limited to: (1) burnable poison rods, (2) clearances or passagesfor axial insertion of control rods or control elements, (3) passagesfor insertions or installation of instruments that measure and monitoreither thermal hydraulics or fission processes (e.g., neutron flux), (4)start up neutron sources, or (6) fluid passages that increase the localliquid water fraction and as such increase neutron moderation. In somelocations throughout the core, including fuel positions at the peripheryof the core where barrier assemblies are inserted, the fuel may notrequire provisions for accommodating any instrumentation, poisons, orneutron sources, as these are “non-instrumented” locations in the core.

Fuel species used in nuclear power reactors are hazardous to the publicand environment if released. Also hazardous are fission “products”produced as a result of the nuclear fission reactions or decayprocesses. These include Cs-137, Sr-90 and Kr-85. Both cesium andstrontium fission products are non-volatile and as such amenable totransport through soil and groundwater.

Nuclear fuel is also highly radioactive and as such is stored underwateruntil such time that fission products have decayed to the extent that itis practicable to handle or store it in air.

In addition to soluble fission products, other soluble and insolublesolid species that circulate in the primary coolant loop include: (1)activated and non-activated corrosion products, (2) metallic impurities(both soluble and particulate), and (3) foreign materials. Byconvention, “soluble” materials in the primary circuit of a nuclearplant are either truly soluble as ionic or non-ionic moieties, or aredefined as those particulates that pass through a filter with a definedrating, for example 0.45 μm. In reality, particulate species may exhibitsizes below 0.45 μm, often with sizes as small as 0.1 μm. Smallerparticles are often termed colloids, acknowledging that as particle sizedecreases, particles begin to manifest some properties of dissolvedspecies. Larger insoluble particulates may exhibit sizes or effectivediameter of up to 8 μm or more.

Corrosion products such as iron and nickel oxides are generated assurfaces of components wetted by the primary coolant oxidize and theoxides are released into the coolant. Soluble metal species such asionic nickel, chromium, cobalt and iron are released from primarycoolant pressure boundary surfaces which may be stainless steel ornickel alloys, or other components such as cobalt bearing valve seats.In the case of BWRs where the primary coolant system may include heatexchangers fabricated with copper-bearing alloys, the soluble speciesmay also include copper.

Metallic fines and particulates particles, exhibiting sizes of order 0.1μm to greater than 8 μm, are released from surfaces due to wear orerosion. Foreign materials include metals, dusts, debris and “foreignobjects” that are left in the primary system after construction andassembly of the plant, or during refueling and maintenance outages whenthe primary system is open to the environment. Foreign objects caninclude metal shavings, tools, fasteners, loose parts and debris.

Concentration of corrosion products in circulation through the core andhence through the fuel assembles is typically on the order of 2 ppb. Fora reactor coolant inventory of 200,000 to 750,000 pounds (100 to 350tonnes), the mass of circulating corrosion products at any one time isquite small, on the order of a gram. Despite such a low inventory incirculation at any one time, each fuel assembly in a PWR core mayaccumulate several hundred grams of crud deposit or other corrosionproduct particulates over the course of an operating cycle. Analyseshave shown that the crud and corrosion products that circulate throughthe core likely pass through the core many times before depositing orplating on the fuel surfaces. Similarly, deposition on ex-core surfacesoccurs slowly over time. At a typical total PWR RCS flow rate of 70 to150 million pounds per hour (32,000 to 68,000 tonnes per hour), the flowof corrosion products through an individual fuel assembly can be severalgrams per day or more, and several kilograms over an 18-month operatingcycle. As such, while fuel assemblies are not efficient filters forcrud, they will eventually capture crud due to various depositionmechanisms including those promoted by a small amount of boiling in thecore and especially as the primary coolant is circulated many thousandsof times through the core.

In addition to the crud on fuel, crud deposits form on essentially allprimary loop pressure boundary surfaces (so called ex-core surfaces).The amount of crud on ex-core surfaces is estimated to be on the orderof a one to tens of kilograms in a PWR, and much higher in a BWR due tothe use of carbon steel in the RCS of a BWR, which has highersusceptibility to corrosion that the austenitic and nickel based alloysused in PWRs.

A specific problem with the foreign objects that circulate with the coreis potential for physical damage to the fuel when even small debrisbecomes lodged near the cladding. Damage mechanisms include wear,fretting and impingement. Numerous methods and apparatus have beendesigned by fuel vendors to capture debris before it can enter the fuelitself. Examples include devices integral to fuel assemblies asdescribed in numerous patents such as U.S. Pat. Nos. 4,664,880;4,684,495; 5,024,807; 5,219,517; 5,390,221; 5,473,649; 5,479,461;5,490,189; 5,524,031; 5,867,551; 6,847,695; 6,901,128; 7,889,829; andPatent Publications US2004/0071255; US2004/0076253; US2005/0031067;US2006/0045231; US2008/0013667and WO2010/076315. Debris filters, trapsor screens are almost always located close to or as a part of the lowernozzle or tie plate in all fuel assemblies, and tend to have relativelylarge flow openings as compared to the size of corrosion products andcrud or they would tend to undesirably restrict flow through the fuelassembly which has negative consequences in terms of reactivity control,heat removal and optimization of core thermal-hydraulic in normal andaccident scenarios. In addition, the debris filters, traps and screensare specifically designed to avoid capture of crud as this could clogand starve of the fuel assembly of water, which would have a potentiallyserious safety impact as cooling of the fuel assemblies is a criticaldesign requirement for the plant. Thus, the flow openings in debrisfilters which are integrated with the fuel assembly itself are typicallygreater than 1 mm in diameter (or equivalent hydraulic diameter).

Corrosion products, metallic fines, and foreign objects can all becomeactivated or rendered radioactive due to neutron absorption as they passthrough or near the core during plant operation. The corrosion productsand fines have often been referred to as “crud” (Chalk RiverUnidentified Deposits, referring to the Chalk River nuclear plant sitein Ontario, Canada). As these materials become activated and spreadthrough the primary circuit, radiation dose fields increase, which inturn increase the radiation exposure or potential for contamination ofworkers. Crud is continuously generated by corrosion or erosion of theprimary circuit. Subsequent activation of the deposits that rest on fuelsurfaces may be released and re-deposit on ex-core surfaces.

Fuel removed after a period of operation typically is coated with alayer of crud. The thickness of this layer may vary from a few micronsto more than 100 microns. In PWRs it tends to be thickest near the topof the fuel assembly, where in some BWRs it tends to be thicker near thebottom of the fuel.

Crud that deposits on fuel in PWRs can lead to a phenomenon known asAxial Offset Anomaly (AOA), also referred to as Crud Induced Power Shift(CIPS), wherein boron that is used for reactivity control accumulates inthe pores of crud and affects the local power density of the core. Loosecrud accumulations on fuel surfaces also can complicate movement of thefuel to dry storage and can complicated transport of the fuel tolocations away from the nuclear plant due to the risk of spreading thehighly radioactive crud to the environment.

Present methods for dealing with issue of crud include: (1)incorporating cleanup systems into the plant, (2) designing fuel that istolerant to crud build up, and (3) developing primary water chemistrycontrol programs that minimize crud generation and maximize release andcapture of crud in exiting plant systems during plant shutdowns, whichis a typical chemistry practice at PWRs. Such “crud burst” evolutionsare expensive in that they delay shutdown of the plant for refueling.

Typically, a PWR incorporates a system in the primary side of the plantcalled the Chemical Volume Control System (CVCS) or “Letdown System.”The primary purposes of the CVCS in PWRS are to: (1) adjust theconcentration of neutron absorbing chemicals in the RCS, typically boricacid, (2) maintain RCS liquid inventory, (3) condition and clean reactorcoolant required as seal water for RCP seals, (4) adjust RCS chemistrythrough addition or removal of species such as lithium, (5) control RCSactivity during power operations by venting noble gases such asradioactive isotopes of xenon and krypton, (6) inert, fill, pressurizeand degasify of the RCS, (7) control RCS hydrogen concentration, and (8)provide a means of adding species that mitigate corrosion of RCScomponents, such as zinc. The CVCS also includes in-line filters(sometimes as separate filters but also sometimes as ion exchange beds,which serve both a filtration and de-ionization function) to remove tothe extent practicable the crud (corrosion products) from the RCS duringstartup, operation and shutdown of the plant. As discussed later, theCVCS operates continuously but is only partially effective as afiltration system.

Reactor coolant typically flows to the CVCS from a RCS “cold leg”letdown line on the suction side of a reactor coolant pump (RCP). In theCVCS, the letdown flow is depressurized, cooled, cleaned, filtered,degassed, equilibrated with desired gases, re-pressurized and reheatedbefore it is returned the RCS or stored.

The normal letdown flow rate is on the order of 16,000 to 32,000 poundsper hour (7 to 14 tonnes per hour) corresponding to a volumetric flow of40 to 80 gallons per minute (13 to 18 cubic meters per hour) at RCStemperature and pressure. As discussed earlier, the normal RCS flow ison the order of 70 to 150 million pounds per hour (32,000 to 68,000tonnes per hour) on a mass flow basis. Thus, the CVCS let down flow ison the order of 0.01 to 0.02% of the RCS flow. With an RCS liquidinventory of about 600,000 pounds (300 tonnes) of coolant, one completeturnover of the RCS through the CVCS takes on the order of 30 hours(1800 minutes). On the other hand, the residence time for recirculatingcoolant in the RCS is less than 1 minute. Therefore, a volume of primarycoolant containing corrosion products passes through the core many timesbefore it passes through the CVCS.

The exact letdown flow is plant specific and may depend on a number offactors including overall RCS design, number of RCS loops, chemistrycontrol strategy and various plant specific goals for chemistry and RCSactivity control. This may include concerns regarding RCS and fuelcladding integrity, as some corrosion mechanisms affecting the RCS andfuel depend on the RCS coolant chemistry such as pH limits, lithium andhydrogen concentration, and zinc concentration. Zinc is often added tothe RCS for mitigation of stress corrosion cracking of certainsusceptible RCS components, or to lower plant dose rates by reducing theincorporation of Co-58 and Co-60 species into ex-core surfaces (i.e.,surfaces of components to which workers may exposed during periods ofrefueling or maintenance).

There are typically several paths by which the letdown flow can beeither returned to the RCS or stored. These include re-injectiondirectly into to the RCS or through the RCP seals. The majority of theletdown flow to the CVCS is typically returned to the RCS through a coldleg of a different loop via a charging line connected to the dischargeside of an RCP.

In the CVCS system, reactor coolant passes through reactor coolantfilters and demineralizers. These filters and demineralizers aredesigned to: (1) remove as much of the insoluble crud and soluble ionicand corrosion product species (e.g., oxides and spinels of Cr, Fe, Ni,Co) as possible, (2) remove excess Li produced by neutron absorption ofB-10, (3) remove non-volatile radioactive fission products such ascesium (e.g., Cs-137) from the RCS, and (4) control boron concentrationfor reactivity control.

The CVCS system incorporates features and components that are designedto permit overall control of the RCS liquid inventory as well.

Generally, the CVCS filters are designed to collect fines andparticulates of 5 micron or greater. Finer filter media can also be usedto collect fines and particulates as small as 0.1 micron. However, thiscan complicate operation of the CVCS system and require more frequentfilter change-outs. In various embodiments, the particle removalapparatus may be adapted for removal of particles in the range of about0.5 to 10 microns, about 1-8 microns, about 1-5 microns, about 50microns and smaller, or about 100 microns or smaller. As will beappreciated, such apparatus will tend to remove particles of the ratedsize or larger. Additionally, devices rated for removal of largerparticles will tend to capture some fraction of smaller particles aswell. Thus, a device rated at 50 microns will also capture some 10micron particles, for example. Furthermore, as particles accumulate, theaccumulation will tend to act to reduce the size of particles that areable to pass through.

In addition, plants that add zinc to the RCS sometimes experience higherparticulates in the RCS and actually tend to use coarser filter media toavoid excessive number of filter replacements. Increasing flow throughthe CVCS to improve filtration above 0.01 to 0.02% of the RCS flow couldin principal be achieved, but this decreases the efficiency of theplant. The letdown flow is cooled in non-regenerative heat exchangers,and only partially re-heated by regenerative heat exchanger; thus, someof the energy in the coolant that passes through the CVCS is lost to theenvironment as opposed to being available to generate electricity.Furthermore, increasing flow through the CVCS may require larger pumpsto re-pressurize the letdown flow before it is returned to the RCS, andmay complicate the other functions of the overall CVCS system such asreactivity control, chemistry control, and RCS volume control.

Evidence of the inability of the CVCS to serve as an effectivefiltration system is widespread. Among the evidence is the following:(1) PWR and BWR fuel becomes fouled with crud over an operating cycle,(2) ex-core radiation doses are primarily caused by crud circulatingthrough and depositing on ex-core surfaces rather than being collectedin the CVCS, (3) estimates of RCS crud inventories are far less than theamount of material collected on RCS filters, and (4) shutdown of theplant can intentionally or unintentionally lead to “crud bursts” in theRCS.

In BWRs, cleanup of the primary coolant is primarily with the ReactorWater Cleanup System (RWCU). The RWCU receives about 300 to 400 gallonsper minute (66 to 90 cubic meters per hour) or 1% of the RCS flow fromthe recirculating primary coolant via letdown flows from the bottom ofthe reactor vessel or the recirculation system. The RWCU system coolsthe letdown flow and uses filters and demineralizers to reduce theinventory of crud in the RCS. Despite higher flow rates as compared toPWR CVCS systems, the amount of residual deposited crud in a BWR RCS istypically orders of magnitude higher than that in a PWR. Most of thiscrud deposits in the core due to the much higher boiling duty of a BWR(although a small amount of boiling also typically occurs in PWR coresnear the upper end of the fuel).

To prevent release of the fuel and fission products from within the fuelrods, designers of nuclear power reactors pay particularly closeattention to the design, fabrication and quality of the nuclear fuel rodcladding to make it as robust as possible. Despite these efforts, fuelcladding integrity issues exist. The severity of fuel integrity issuesis well documented in “The Path to Zero Defects: EPRI Fuel ReliabilityGuidelines” (2008). As discussed in this reference, fuel reliability iscritical to the safe and economical operation of nuclear power plantsand the cost of fuel failures due to even minor breaches in the claddingof a limited number of fuel rods can cost a utility as much as $40 to$80 million dollars.

Furthermore, owners and operators of nuclear plants expend great effortto “design the core” so as to place once, twice and thrice burned fuelin a pattern that optimizes energy production, but minimizes thepotential for crud deposition and hence cladding corrosion or cruddeposition.

The designers and operators of nuclear power plants strive to maintainprimary coolant chemistry and minimize impurities through primarychemistry control programs. Objectives of primary water control include:(1) maintaining chemistry and pH, (2) controlling concentration of crud,(3) reactivity control under normal and accident conditions. Duringshutdown, the chemistry of the RCS coolant can be adjusted to promotethe release of soluble and insoluble species from in core (i.e. fuelsurfaces) or ex-core (i.e. RCS) surfaces. This is typically done bysequentially producing chemically reducing followed by chemicallyoxidizing conditions as the RCS is cooled. During such crud bursts, ithas been estimated that from about 100 to several thousand grams of crudis dislodged from ex-core surfaces and collected in the CVCS over aperiod of about 24 hours as the plant cools down. This compares to the10's of thousands of grams (or 10's of kilograms) of crud that areresident in the RCS prior to shutdown of the plant. In this regard, theparticle removal apparatus in accordance with embodiments may be adaptedto remove 100 or hundreds of grains, or may be adapted to remove 1000 or10000 grams, or ranges within those endpoints.

Owners and operators of nuclear plants are faced with two critical tasksthat relate to presence of corrosion products, impurities, fission andfuel fragments, and foreign materials in the primary coolant system. Thefirst is to minimize the potential for fuel failures; the second is tomanage radiation fields. Removal of these species is also desirable asit can reduce the radiation emitted from the primary coolant systemduring periods of plant maintenance.

The economic significance of maximizing fuel reliability, reducingradiation doses, and optimizing the safe and economic operation of theplants is high. Specifically, the cost of extending a refueling outageby even one day to accommodate a “crud burst” may exceed $1 million. Aforced outage due to failed fuel can exceed $10 million. The cost ofreducing power because of crud induced AOA can exceed tens of millionsof dollars.

The economics of significance of reducing worker radiation dose is alsosignificant. For example, a reduction in worker dose in an outage by 1man-rem is assigned a “value” of $10,000 to $30,000. If a typical outagedose is 100 man-rem, a 25% reduction in worker radiation exposure isequivalent to $250,000 to $750,000.

One aspect of an embodiment of the invention is to reduce the inventoryof crud circulating in the RCS of a nuclear plant and thereby thedeposition of crud on fuel and on ex-core surfaces.

Another aspect of an embodiment of the invention is to collect the crudsuch that it can be removed from the RCS during refueling outages.

Another aspect of an embodiment of the invention is to achieve reducedinventories of circulating crud with moderated impact on the electricaloutput of the plant while avoiding or reducing embrittlement of thereactor pressure vessel or other potential risks to plant integrity.

Embodiments of the present invention relate to methods and apparatusesfor capturing particulates and crud from the RCS fluid in a nuclearpower plant. One or more particulate removal apparatuses are positionedin the core of an electric power producing PWR or BWR plant. The plantis operated with the apparatus in place. The particulate removalapparatus may replace one or more fuel assemblies, or be a modified fuelassembly with integral filtration or other means of particulate capture(e.g., a hydrocyclone or combination of filtration and hydrocyclones).In this regard, the apparatus is dimensioned substantially similarly toa fuel assembly and includes connectors that are similar to those in atypical fuel assembly to allow it to be placed in a fuel assemblyposition without modification to the core. As will be appreciated,substantially similarly in this context means that the dimensions andconnectors are sufficiently identical to those of a fuel assembly sothat the apparatus may be positioned appropriately.

The particulate removal apparatus can be placed at any core location. Inan embodiment, the particulate removal apparatus is placed at theperiphery of the core. In a further embodiment, the apparatus may beplaced in a location normally reserved for a barrier fuel assembly. Byplacing the apparatus in a location normally used for a two-times orthee-times burned barrier assembly, there can be a reduction in possibleloss of power generation during the operating cycle, since the fuelnormally loaded in these locations would have contributed less to plantoutput than the average fuel assembly. In another embodiment, one ormore pairs of particulate removal apparatuses are placed in symmetricallocations in the core to promote the overall neutronic and thermalhydraulic symmetry of the core. In another embodiment, the overall “coredesign” and the remainder of the fuel assemblies are designed tocompensate for any reduction in local power generation at thelocation(s) of the apparatus, for instance by small increases in theenrichment of other fuel assembles.

The overall thermal hydraulics of the core and loads on core supportstructures are maintained when employing the apparatus since it isdesigned so that the reactor coolant fluid flow through the apparatus issimilar to that of a normal fuel assembly. The apparatus is alsodesigned so that pressure drop of the coolant as it passes through theapparatus is similar to that of a normal fuel assembly. Morespecifically the flow through the apparatus is similar to that of anormal fuel assembly or about 0.5% of the core flow, and thecross-sectional area and form drag and friction loses in apparatus aresimilar to those in a fuel assembly. While specific embodiments of theinvention may involve using more than one apparatus, use of just oneapparatus is possible, as the flow through one apparatus is about 25 to50 times higher than the flow through the CVCS in a PWR. Further, theuse of multiple apparatuses would further enhance the proportion offiltered flow (i.e., the flow rate through two filter apparatuses wouldbe 50 to 100 times higher than through the CVCS in a PWR and so on).Hence, the potential of the apparatus serving as a means of filtrationfor a given time period is greatly enhanced over the CVCS at equivalentfiltration efficiencies.

FIG. 1 illustrates a typical PWR primary coolant loop showing a reactorvessel 1 and core 2, pressurizer 3, steam generator 4, reactor coolantpump 5, CVCS 6, and charging pump 7.

Equation 1 describes a simple model of the filtration efficiency of theinvention:

$\begin{matrix}{A = {{ST}\frac{\left( {N - N_{c}} \right)\eta_{a}}{{\left( {N - N_{c}} \right)\eta_{a}} + {N_{c}\eta_{c}} + {\frac{F_{l}}{F_{t}}N}}}} & \lbrack 1\rbrack\end{matrix}$

-   -   where    -   A=core crud accumulation on fuel assemblies (kg/cycle)    -   S=crud generation rate (kg/s)    -   T=cycle length (s/cycle)    -   N=number of locations for fuel assemblies in the core (the total        of those occupied by fuel assemblies and those occupied by        filtration apparatuses)    -   N_(c)=number of filtration apparatuses in the core    -   η_(a)=fuel assembly crud removal efficiency    -   η_(c)=filtration apparatus crud removal efficiency    -   F_(l)=the flowrate of coolant to the CVCS or RWCU letdown (kg/s)    -   F_(t)=the flowrate of coolant through the core (kg/s)

FIG. 10 shows an example of the potential for particulate crud/corrosionproduct capture even with an assumption that removal by the letdownsystem is 100% efficient. Even at a conservative efficiency of 80%, theability to reduce core crud inventories by a factor of three or more ispredicted. With four apparatuses, the reduction may be about 90%, orfrom 3 kg without the apparatuses to about 0.3 kg with the apparatuses.

In an embodiment, the apparatus included filter media that may be finerthan that used in a typical CVCS (on the order of 5 micron). Forexample, on the order of 100 ft² of 0.1 to 0.5 micron rated porous metalfilter media will allow for a flow equivalent to that which flowsthrough a normal fuel assembly and will result in a pressure drop on theorder of 10 to 20 psi, with the later pressure drop occurring afterretention of several kilograms of particulate material (an inventorythat is similar to that which exists in PWR RCS system). This pressuredrop range is similar to or lower than that seen through each fuelassembly in the core, and features are incorporated into the filtrationapparatus to match the pressure drop even more closely to that of anormal fuel assembly. More specifically, additional flow restrictionssuch as orifices may be incorporated into the apparatus to tailor thepressure drop to be even closer to that of a normal fuel assembly. Aswill be appreciated by those skilled in the art, filter media withlarger openings may also be incorporated into a particulate removalapparatus of the current invention, for example, to remove particles upto 10 micron in diameter or agglomerations of particles to 100 micron ineffective diameter. These agglomerations may be comprised of exfoliatedcrud particles, for example.

In an embodiment, the particle collection apparatus includes a passiveflow-restricting component that reduces flow through the particlecollection apparatus in response to changes in water temperature, flowrate, pressure, density, viscosity, or other local condition that variesas a function of plant status. Such flow-restricting components include,but are not limited to, bimetallic disc valves, spring-loaded reliefvalves, check valves, foot valves, labyrinth seals, and orifices. Suchflow-restricting components generally tend to improve the retention ofcaptured particles times when particle removal is not required, such asduring plant shutdown and forced oxidation (crud burst) evolutions.

Other features can optionally be incorporated in various embodiments ofthe invention including debris filters or isolation valves to preventthe escape of foreign material from the apparatus. As discussed above,debris filters which are typically integrated into the design of nuclearfuel assemblies must have a minimum opening diameter to ensure adequatecooling flow to the fuel assembly. In contrast, debris filtersincorporated into a particulate removal apparatus of the type disclosedherein may be comprised of wide range of opening sizes because theparticulate removal apparatus is separate from the fuel assembly. Forexample, debris filters incorporated as part of the current inventionmay have openings that are up to 0.1 mm in effective diameter, whichwould be expected to trap small metallic fines and other small foreignobjects that would pass through the requisite openings in standard fuelassembly debris filters. Debris filters incorporated as part of thecurrent invention may also have openings that are 1 mm or greater ineffective diameter in order to capture larger particles and debris.

In an embodiment, corrosion resistant and radiation tolerant materialsmay be used for the construction of the apparatus including anyfiltration media in the apparatus. One example is a porous metal filtermedium. Low cobalt metallic filter media may be used to reduce potentialcontribution of radioactive species to the RCS from the filter itself.

In an embodiment, passive metal mass is included as part of theapparatus to not only replicate the weight of a fuel assembly (owing tothe particulate capturing component of apparatus such as the filtermedia likely being less weight that fuel rods), but also provideshielding capability much like a barrier fuel assembly. This mass ofmetal may be in the form of plates or a plurality of solid rods.

In an embodiment, upper and lower nozzle designs are replicated so thathandling and storing of the apparatus with existing plant equipment suchas refueling machines and spent fuel pool racks.

In an embodiment, a bypass flow path is included to ensure coolant waterstill flows through the apparatus even in the event that the filterclogs with crud.

In an embodiment, the particulate capturing components of the apparatusare incorporated in a portion of a fuel assembly containing a pluralityof normal fuel rods, but a reduced number as compared to a typicalassembly. In this embodiment, the hybrid fuel assembly/filtrationapparatus serves both as a filter and as a source of nuclear energy.

In an embodiment, the particulate removal apparatus may be removed fromthe nuclear reactor and the particulate removal capacity may beregenerated. This removal may occur, for example, during a normalmaintenance outage and regeneration may be achieved, for example, bybackwashing the particulate removal apparatus or by using ultrasoniccleaning techniques.

In an embodiment, the particulate removal apparatus may be handled,stored and disposed of in the same manner as a nuclear fuel assemblywhen its particulate removal capacity is exhausted. For example, theexhausted particulate removal apparatus may be transferred to and storedin the spent fuel rack.

FIG. 1 illustrates an embodiment of a typical PWR primary coolant loopshowing a reactor vessel 1 and core 2, pressurizer 3, steam generator 4,reactor coolant pump 5, CVCS 6, and charging pump 7.

FIG. 2 is a plan view of an embodiment of a reactor vessel core,comprised of a grid of fuel assemblies within a circular envelope. Oneor more fuel assemblies near the outer edges of the core 8, referred toas barrier assemblies, may be installed in the indicated locations toprovide shielding to the reactor vessel 9 from radiation from higherpower assemblies closer to the center of the core. In anotherembodiment, pairs of particulate removal apparatuses are installed insymmetric location of the core such as 9 a or 9 b. One, two or more ormore pairs of apparatuses may be utilized.

The exact location each of the individual apparatuses constituting apair may depend on the exact core design which would take into accountlocal neutronics, overall core neutronics, the location of coreinstruments and control rods, as well as the physical symmetry of thelocations. As will be appreciated by those skilled in the art, multipleparticulate removal apparatuses may be used in symmetric ornon-symmetric configurations. The exact number of apparatuses to be usedand installation locations would be dependent on the exact core design.

A more particular embodiment is shown in FIGS. 3 to 9, including aparticulate removal apparatus with outside profile similar to that of afuel assembly.

FIG. 3 is a plan view schematic of an embodiment of a particulateremoval apparatus, including a housing 10, particulate removal zone 11,and bypass hole 12.

FIG. 4 is an elevation view of an embodiment of a particulate removalapparatus with a single zone of particulate removal. An upper nozzle 13is connected to a lower nozzle 14 by a housing 10. A foot valve 15 islocated at the entrance to the particulate removal 11.

FIG. 5 is an elevation view of an embodiment of a particulate removalapparatus with multiple individual zones of particulate removal 16within the overall particulate removal zone 11.

FIG. 6 is an elevation view of an embodiment of a particulate removalapparatus a multiple zones of particulate removal within the particulateremoval region 11 of different types, including a cyclonic separationstage 17 and a conventional filtration separation stage 18.

FIG. 7 is an elevation view of an embodiment of a particulate removalapparatus the bypass flow path 19 through the module.

FIG. 8 is a plan view of an embodiment of a hybrid particulate removalapparatus includes a particulate removal zone 11 within an array ofnuclear fuel rods 22.

FIG. 9 is an elevation view of an embodiment of a particulate removalapparatus with one or more particulate removal features 16 extendingover a partial length of the apparatus integral to the upper nozzle 13or lower nozzle 14.

Flow enters the assembly through a lower nozzle 14, and is divided intoa particulate removal flow and bypass flow 19. The particulate removalflow is directed through an optional foot valve 15, through aparticulate removal region 11, and then out of the assembly through anupper nozzle 13.

The foot valve 15 is designed such that it is fully open during poweroperation (high core flow, high temperature), but closed during shutdowncrud burst evolutions (low core flow, reduced temperature), such thatthe dissolution of captured crud during crud burst evolutions isminimized. In addition, the foot valve prevents captured crud fromfalling out of the assembly during handling or storage.

The bypass flow 19 is directed from the lower nozzle region through anorifice 12 bypassing the foot valve 15 and filtering region 11, andjoins the particulate removal flow through the assembly and out throughan upper nozzle 13.

Portions of the assembly integral to or adjacent to the housing 10between the lower and upper nozzles 13 and 14 are comprised of thickmembers that function as the structure of the assembly, provide neutronshielding for the vessel similar to a barrier assembly, and contributemass so that the assembly mass is similar to that of a fuel assembly.

The particulate removal region 11 may be comprised of one or more zonesof filtration, incorporating one or more different filtration orseparation processes. In one embodiment, the filtering region 11 iscomprised of one or more filter elements made from sintered metal fiber,sintered metal powder, wedge wire, wire mesh, or other radiationtolerant media.

Another embodiment is shown in FIG. 6, in which the particulate removalregion 11 includes both cyclonic separators 17 and filter elements 18.In this embodiment, flow through the cyclone separators is divided intoa high particulate concentration “underflow” 21 and a low particulateconcentration “overflow” 20. The underflow 21 is directed through thefilter elements 18 and then exits the assembly through the upper nozzle13. Some or all of the overflow 20 bypasses the filter elements and isdischarged out of the upper nozzle 13.

The invention is designed to be handled and disposed of in the samemanner as spent nuclear fuel. The geometry of the upper 13 and lowernozzle 14 is designed to mimic that of a fuel assembly for handlingpurposes, and to interface with the appropriate reactor vesselcomponents.

In the embodiment illustrated in FIG. 7, the bypass flow path 19 isdesigned to control pressure drop across the assembly, provide coolingflow in the event of complete filter media plugging, and to provide aflow path for draining of water from the assembly for disposal.

The invention may also be adopted for use in pressurized heavy waterreactors (PHWRs) or Eastern European PWR designs such as VVER plants.

In an embodiment, the particulate removal apparatus is handled, storedand disposed of in a similar manner to a fuel assembly. In this regard,after use, the particulate removal apparatus may be placed in a spentfuel rack after its particulate removal capacity is exhausted.

In embodiments, the particulate removal capacity is regenerated.Regeneration may be performed, for example, by backwashing theparticulate removal apparatus or by ultrasonic cleaning, though otherapproaches are available. It may be useful to perform regenerationduring a normal maintenance outage, so that the regeneration functiondoes not reduce plant availability.

In embodiments, the particulate removal apparatus may incorporate aplurality of particulate removal zones within a particulate removalregion. The plurality of zones may be arrayed in series or in parallel.

In an embodiment, a passive flow limiting device is used to restrictflow past the collected particles during specific plant operatingconditions to improve the retention of captured particles.

As will be appreciated from the foregoing, core designers can optimizethe enrichment and loading of other fuel assemblies so as to compensatefor the loss of energy production resulting from the removal of one ormore fuel assemblies from the core and replacing them with the describedparticulate removal apparatus. As discussed earlier, placing theapparatus at the location of a barrier assembly would tend to result inreduced penalty with respect to energy production, as those assembliesare essentially depleted. Placement of pairs of apparatuses insymmetrical locations in the core may simplify core design and result insymmetric thermal hydraulics and neutronics.

Those skilled in the art will appreciate that the disclosed embodimentsdescribed herein are by way of example only, and that numerousvariations will exist. Where not otherwise understood by those ofordinary skill in the art, the terms “substantially” or “about” shouldbe understood to encompass differences of approximately 10%. Theinvention is limited only by the claims, which encompass the embodimentsdescribed herein as well as variants apparent to those skilled in theart. In addition, it should be appreciated that structural features ormethod steps shown or described in any one embodiment herein can be usedin other embodiments as well.

The invention claimed is:
 1. A method for removing corrosion productparticulates from recirculating primary coolant in a nuclear plantcomprising: operating the nuclear plant to generate power; collectingand capturing corrosion product particulates generated by corrosion ofprimary circuit wetted surfaces, by filtering the recirculating primarycoolant with a particulate removal apparatus located in a core of thenuclear plant while operating the nuclear plant to generate power, theparticulate removal apparatus including a porous metallic filter mediumadapted to filter and capture the corrosion product particulates in thecore of the nuclear plant while coolant fluid recirculates in the coreof the nuclear plant to improve fuel integrity and performance bypreventing the corrosion products from depositing on fuel claddingsurfaces and to mitigate radiation fields within the nuclear plant bypreventing the release and redistribution of activated corrosionproducts, the particulate removal apparatus being installed in the corein place of at least a part of a standard fuel assembly; and restrictingflow through the particulate removal apparatus in response to a changein a coolant chemistry to retain captured particulates; wherein theparticulate removal apparatus is removed from the core after oneoperating cycle.
 2. The method of claim 1 wherein the particulateremoval apparatus is installed in a barrier fuel location.
 3. The methodof claim 1 wherein the particulate removal apparatus is installed in acenter of the core.
 4. The method of claim 1 wherein pairs ofparticulate removal apparatuses are installed in symmetric locations. 5.The method of claim 1 wherein the core design is adjusted to compensatefor the lost energy production from the fuel that would have beeninstalled at the location used for the particulate removal apparatus. 6.The method of claim 1 wherein the nuclear plant is selected from thegroup consisting of a PWR, BWR, a CANDU or a VVER.
 7. The method ofclaim 1 wherein the filter medium comprises a radiation tolerantfiltration medium adapted to be tolerant to radiation present in thecore of the nuclear reactor.
 8. The method of claim 1 wherein theparticulate removal apparatus further comprises a cyclone separator, thecyclone separator being adapted to produce a first flow having a lowerparticulate concentration and a second flow having a higher particulateconcentration, the method further comprising directing the first flowthrough a bypass path and directing the second flow through the filtermedium.
 9. The method of claim 1 wherein particulate removal capacity ofthe particulate removal apparatus is regenerated.
 10. The method ofclaim 1, wherein the particulate removal apparatus includes a bypasspath, configured to allow fluid flow to bypass the filter medium, andwherein the method further comprises opening flow through the bypasspath to reduce dissolution of captured crud during a crud burst event.11. The method of claim 1, wherein the filter medium is configured andarranged to filter particulates having a diameter between 0.1 μm and 8μm.
 12. The method of claim 1, wherein the particulate removal apparatuscomprises a filtration medium with filtration pores sized to removeparticulates in the range from about 1 μm to about 5 μm.